Casting process and apparatus for obtaining unidirectional solidification



Sept. 7, 1965 Filed Oct. 24, 1960 M. c. FLEMINGS, JR. ETAL UNIDIRECTIONAL SOLIDIFICATION 2 Sheets-Sheet 1 -WATER 3 FUEL WATER LGAS 4-..-

EXOTHERMIC MATERIAL FIG. I s

COPPER WATER- L 4 I SAND PROGRAMMER FIG.3

AIR GAS CONTROL CONTROL |l i Eii i I I l l COOLING AIR INVENTORS MERTON C. FLEM|NGS,JR HOWARD F. TAYLOR ATTORNEYS Sept. 7, 1965 M. c. FLEMINGS, JR., ETAL 3,204,301

CASTING PROCESS AND APPARATUS FOR OBTAINING UNIDIRECTIONAL SOLIDIFICATI N Filed Oct. 24, 1960 o 2 Sheets-Sheet 2 EXOTHERMIC MATERIAL INGOT 28 .G 22 COPPER I 20 FIG. 2

9o as PROGRAMMER WATER l FIG. 4

WATER CONTR OL POWER CONTROL 68 WATER II/I INVENTORS MERTON C.FLEMINGS,JR. BYHOWARD F. TAYLOR ATTORNEYS United States Patent 3,204,301 CASTING PROCESS AND APPARATUS FOR 035- TA'INING UNIDIRECTIONAL SOLIDIFICATION Merton C. Flemings, .112, Lexington, and Howard F. Taylor, Belmont, Mass, assignors, by mesne assignments, to M. 'C. Flemings, Jr., and Edith L. Taylor, executrix of said Howard F. Taylor, deceased Filed Oct. 24, 1960, Ser. No. 131,700 6 Claims. (Cl. 22-74) (Filed under Rule 47(a) and 35 U.S.C. 116) This invention relates to methods for casting metal ingots and more specifically to methods wherein the cooling is carefully controlled to optimize properties of the cast ingot.

In conventional ingot-making processes, liquid metal is poured from ladles into metal or refractory molds at the conclusion of melting and deoxidation processes. With fully killed steels, the resulting ingot consists of a surface of fine chill grains, a columnar zone of crystallization along the mold walls, a central zone containing substantial amounts of non-metallic inclusions and porosity, and a pipe or large shrinkage void in the region that was last to solidify. Generally, ingot molds are designed to minimize the length of the pipe since this portion of the ingot must be removed and discarded as scrap. The mold is usually of cylindrical or square cross section with varying arrangements of taper or flutes on the vertical faces. Through the use of such tapering, fluting, and hot topping, an attempt is made to concentrate metal shrinkage in one portion of the ingot. However, even with the use of all the conventional techniques, a substantial fraction of the killed steel ingot must be discarded.

In addition, since the central zone of the casting is not removed, further working of such ingots in conventional rolling, forging, or other processing distributes the porosity and inclusions through the final product. Long lines of inclusions in rolled stock known as stringers arise in this way.

It is, therefore, an object of this invention to produce metal ingots with increased ratio of the weight of usable metal to the total weight poured in an ingot mold.

A further object is to produce an ingot with increased chemical homogeneity.

A further object is to produce ingots of improved soundness.

A further object is to produce ingots with a minimum of non-metallic inclusions.

A still further object is to segregate chemical impurities in the portion of the ingot to be discarded.

These and other objects are achieved in a process wherein most of the heat transfer during the cooling of the casting takes place through one surface, resulting in unidirectional solidification. More particularly, the process involves maintaining all surfaces of the mold except one at substantially the temperature of melting steel and removing heat through the one remaining surface.

FIGURE 1 is a cross-sectional view of a mold adapted for use with the process of this invention and employing exothermic material.

FIGURE 2 is a cross-sectional view of a mold adapted for use with the process of this invention employing exothermic material.

FIGURE 3 is a cross-sectional view of a mold adapted for use with the process of this invention employing gas heating.

FIGURE 4 is a cross-sectional view of a mold adapted for use with the process of this invention employing electrical heating.

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Referring to FIGURE 1, a mold suitable for use with the process of this invention is shown. The main inner body 2 of the mold is composed of exothermic material. Exomold E, a proprietary compound sold by Exomet, Inc., of Conneaut, Ohio, for use if steel is to be cast, is a suitable material.

Exothermic materials suitable for use with steel are normally composed of iron oxide, aluminum and a binder such as sand. An effective burning temperature of about 3,000 F. and a structurally stable composition results from such mixtures. The effective burning temperature can be made to match other casting materials by varying the percentage of binder used and by using other ingredients, such as magnesium.

The bottom inner portion 4 of the mold is formed of a copper block. Water for cooling block 4 is supplied through lines 6 and 8. The inner mold pieces are placed within the sand 10 of a conventional mold box 12.

The casting steps are as follows:

First, the molten metal, steel for example, is poured into the cavity formed by the inner mold pieces 2 and 4. Then an exothermic cover 14 is placed on top of inner mold piece 2. The exothermic material of which inner mold pieces 2 and 14 are composed reacts and burns when contacted by the molten steel contained within the cavity. Therefore, the exothermic material provides an essentially perfect insulator for the inner surfaces of mold pieces 2 and 14. As a result, there is substantially no heat flow in either direction through the surfaces. The remaining mold surface 16 of copper block 4 provides, therefore, substantially the only path for the removal of heat from the molten steel within the cavity.

With heat being withdrawn only through surface 16, the molten steel within the cavity solidifies upwardly through the cavity. While it is customary to enlarge the upper portion of an ingot mold to minimize any pipe remaining within the casting, this enlarged portion of the cavity is unnecessary with the process of this invention. In fact, the central void is so effectively reduced that if desired a smaller, rather than larger, upper central section can be employed to ensure more complete filling of the mold by the cooled ingot 18. Normally, however, it will be simpler to design the mold slightly taller than the desired ingot to allow for the shrinkage upon solidification and cooling which occurs with most materials.

Another mold configuration utilizing exothermic material is shown in FIGURE 2. In the embodiment of FIGURE 2, the interior diameter of the mold is uniform. The exterior, however, is formed of a series of rings 20, 22, 24, 26 and 28 whose diameters increase with their distance away from the cooling surface 16. Thus, in the areas requiring temperature isolation for the longest period of time, a greater amount of exothermic material is provided. Instead of the series of rings, a single piece of varying thickness could be used. The casting operation is identical to that described in connection with FIGURE 1. That is, substantially unidirectional cooling is obtained, the heat being extracted through surface 16 by water-cooled copper block 4.

When exothermic material is used for the mold surfaces, somewhat improved surface texture and purity in the final ingot can be obtained if the exothermic material is pre-ignited. In the above descriptions of casting operations employing the structures of FIGURE 1 and FIG- URE 2, the molten metal itself was used to ignite the combustible components of the exothermic materials. Thus, the combustible components are, at least momentarily, burning in contact with the molten metal being cast. When pro-ignition is utilized, a gas flame or other high temperature medium is used to ignite the combustible components of the exothermic material shortly before the molten metal is poured into the mold. While provided in FIGURE 3 for a different purpose, an arrangement such as that of nozzles 46 can serve to preignite the exothermic material.

An alternative structure is shown in FIGURE 3. In this embodiment, the mold comprises a cylindrical portion of conventional refractory material 30 and a refractory top 32. The bottom surface is again provided by a copper block 4 with Water cooling through lines 6 and 8. However, unlike a conventional mold, pieces 30 and 32 are provided with a source of heat. Gas is fed through line 34 to valves 36, 38, 40 and 42. These valves supply nozzles 44 and 46 which play flames on the entire exterior surface of pieces 30 and 32.

The sequence of operations for casting'is similar to that for the structure of FIGURE 1. The molten steel is first poured within the mold cavity. Then the cover 32 is placed over the mold. The nozzles 44 and 46 are supplied with gas and ignited to produce a temperature on the exterior surfaces of mold pieces 30 and 32 which almost equals that of the molten steel within. Since the inside and outside temperatures of pieces 30 and 32 are substantially identical, little heat transfer takes place through these portions of the mold. The bottom, however, composed of surface 16 of copper block 4 is cooled by the water supplied by lines 6 and 8. Therefore, as before, substantially all the heat transfer takes place through the lower surface and the steel solidifies from surface 16 upwards.

While satisfactory castings may be obtained from the structure of FIGURE 3 utilizing only those portions described above, increased control of the casting process is possible if additional control means are used. Valve controller 48 is provided to control the gas valves and valve controller 50 is provided to control air valves. Programmer 52 determines the operation of controllers 48 and 50 according to a predetermined sequence. The sequence of casting steps utilizing the controllers is as follows.

First, the mold, except for the copper block 4, is brought to approximately the temperature of the metal being cast. To accomplish this heating, the programmer 52 directs the valve controller 48, just prior to pouring the metal, to open valves 36, 38 and 40 to provide flames from nozzles 44 to heat mold piece 30 to approximately the temperature of the metal being cast. At the same time, programmer 52 has directed air controller 50 to maintain all air valves closed. Next, the metal is poured and cover 32 secured in place. The water cooling of surface 16 causes the metal in the lowermost portion to solidify. Solidification continues in a uni-directional manner away from surface 16.

Then, as the cooled and solidified zone passes up the cylindrical casting, the gas input to nozzles 44 below that zone is terminated and cooling is provided through air nozzles 64. While sensing means such as thermocouples could be provided to determine the location of the zone of solidification, a programmed sequence for reducing heat input and introducing cooling gives satisfactory results.

An equation which has been found to govern solidification in a unidirectionally solidified ingot is:

where x equals the distance in inches from the chill face to the end of the completely solid metal, k equals a constant dependent primarily on the particular metal being cast, 1 equals the time in minutes, and C is another constant. Thus, once constants have been determined by experiment for a particular metal, the sequencing of operation for controllers 48 and 50 can be determined for optimum operation. For steel, k is approximately 1, and C is of the order of /2 to 1.

Programmer 52 is furnished with a predetermined program to determine the operation of gas controller 48 and air controller Stlwith time. If steel is being cast, gas controller 48 will be ordered to close valves controlling nozzles 44 Within approximately the square root of the time in minutes, plus approximately 1 inch from bottom surface 16 of the mold. Thus when approximately /3 of the ingot has solidified, the programmer 52 would order gas controller 48 to close valve 36. At the same time, programmer 52 would direct air controller 50 to open valve 56 to admit cooling air from line 54 to the lowermost nozzles 64. As the zone of solidification progresses upwards through the ingot, the programmer 52 directs the gas controller 48 to close progressively more gas valves. At the same time the, programmer directs air controller 50 to open a correspondingly increasing number of air valves. While the extraction of heat from surfaces below the cooled zone means that the cooling is not completely unidirectional, it is substantially so and the total casting time is decreased due to the increased rate. of heat extraction.

FIGURE 4 illustrates another embodiment for practicing this invention in which greater control of the casting process is also available. The lower surface 16 of the mold is, as before, one face of a copper block 4. The cylindrical portion 30 and the cover 32 are composed of refractory materials, as in the embodiment of FIGURE 3. In the present embodiment, however, heating for these members is supplied by means of induction heating coils 66 and 68 in close thermal contact with cylindrical portion 30 and cover 32 respectively. The induction heating coils are hollow and are provided with cooling water from line 70 under the control of valves 74, 76, 78, and

The sequence of casting steps with the embodiment of FIGURE 4 is as follows: First, before pouring is begun, the programmer 86 directs the power controller 84 to apply power from source to induction heating coil supply lines 92, 94 and 96. At the same time, the programmer 86 directs water controller 82 to open valve 72 which supplies cooling water to the copper block 4. Thus, prior to pouring, the cylindrical mold portion 30 is heated to approximately the temperature of the metal to be cast. The cooling water applied to the copper block forming lower face 16 of the mold is likewise applied before pouring to prevent damage to the block.

Next, the metal is poured and the cover 32 secured in place. The operation with respect to the ingot itself is similar to thatpreviously described in connection with FIGURE 3. In other Words, the water cooling of surface 16 causes the metal in the lower-most portion to solidify. Solidification continues in a unidirectional manner away from surface 16. Below the zone of solidification the heating is terminated and cooling begun.

In FIGURE 4, induction heating and water rather than gas and air are utilized for heating and cooling respectively. As the zone of cooling progresses through the ingot, the programmer 86 directs the power controller 84 to terminate the application of electric power from source 90 to the induction heating coils 66 below the cooled zone. At the same time, the programmer 86 directs water controller 82 to increase the flow of cooling water through all coils surrounding solidified ingot. For example, if /3 of the ingot is solidified, programmer 86 will direct power controller 84 to terminate the application of power from source 90 to those induction heating coils 66 supplied by line 92. At the same time, the programmer 86 will direct water controller 82 to increase the flow of water through valve 74, thus applying additional cooling to those coils through which induction heating is no longer being applied. The Water flow through the remaining valves 76, 78 and 80 will continue to be only that necessary to keep the induction heating coils from melting.

Although the above descriptions have been with reference to embodiments incorporating material particularly well-suited for casting steel ingots, the processes are applicable to the casting of other metals as well. While the methods are particularly advantageous for killed steel ingots, ingots of increased soundness and chemical uniformly are also obtained with other metals. The particular exothermic material or heating temperatures involved would be chosen to correspond with the melting point of the particular metal being cast.

By way of example, an ingot of steel was cast utilizing an exothermic mold and unidirectional cooling. The particular alloy employed was AISI 4340 steel. Exomold E, a proprietary compound of the Conneaut Company, was used, and the diameter of the ingot in contact with the cooling surface was four inches. An examination of a cross-section through the length of the ingot disclosed extremely uniform structure. The grain structure of the casting was composed almost completely of long columnar dendrites growing perpendicular to the chilled face. Ingots with long columnar grains possess advantagous structural characteristics.

Chemical analysis confirmed the apparent uniformity of the AISI 4340 ingot. The following Table I gives the percentages of various elements in the composition at six distances from the chilled surface 10, the distances ranging from 4 inch to 7% inches.

Table I Distance from Composition Element (weight percent) chilled surface (inches) 0 Mn P S S1 Cr N1 M0 It will be noted that the percentages of carbon, manganese, phosphorus, chromium, nickel and molybdenum are extremely uniform throughout the entire range. In the case of the harmful elements sulphur and silicon, however, there has been a substantial concentration of these elements in the portion furthest removed from the cooling surface While the reason for the centration is not known for certain, careful examination of the casting indicates that the impurity elements formed non-metallic inclusions which floated up from the bottom of the ingot during solidification. Thus, in addition to the increased soundness and uniformity produced by unidirectional solidification, non-metallic inclusions which are deleterious to the mechanical properties of ingots and parts made from ingots, can be floated to the uppermost portion of an ingot during solidification. This upper portion of the ingot can then, if desired, be cut off and later remelted and purified.

Table 11 sets forth the mechanical porperties of test bars cut from A181 4340 steel ingots produced as described above.

It will be noted that the tensile strengths, both transverse to the grains and longitudinal with the columnar grains, are over 290,000 lbs. per square inch, an exceptionally high value for this alloy. Again, with regard to both transverse and longitudinal orientations the yield strength, elongation, and reduction in area are all values exceptionally favorable for castings of this alloy. While the particular figures obtained would be different with different alloys, castings of uniformly high quality can be obtained.

Economicconsiderations or availability may dictate a choice of. materialsdifl'ering from those described above. Withregard to those. portions composed of exothermic material, any available material can be used so long as its temperature characteristics substantially match those of the metal being cast. Similarly, where the mold walls are formed of refractory material, any material suitable for use at the temperatures involved may be used. Since substantially no heat flow from the molten material is involved, conductivity is not important, except for embodiments applying cooling to all non-heated surfaces. In those embodiments, metallic molds are advantageous when compatible with the temperature of the metal being cast. While copper is a preferred material for the main cooling surface, any material of good conductivity and stability at the temperature involved may be used. In fact, at the temperatures and heat flows involved with casting steel, steel is substantially the equivalent of copper for block 4.

While heat application by means of gas flames and induction heating coils has been shown, heat could be applied in any conventional manner, such as by resistance heating or radiant heating. In embodiments employing programmed heating, a minimum number of valves and control lines has been shown for clarity in the drawings.

Those skilled in the metallurgical arts will recognize that considerable variation can be made in specific means involved without departing from the scope of this invention of unidirectional casting.

Having thus described our invention, we claim:.

1. A mold for use in casting a steel ingot, comprising fixed bottom and side walls and a separable top wall completely enclosing a mold cavity therebetween, said bottom wall being formed of heat conductive material, said side and top walls being formed of imperforate exothermic material which extends continuously over the sides and top of said cavity when said top wall is in place, whereby molten steel may be poured into said mold with the top wall removed and then said top wall may be set in place to seal said mold cavity, the combustion of said exothermic material preventing loss of heat through said top and side walls to ensure unidirectional solidification of said ingot as heat is withdrawn through said bottom wall.

2. The mold of claim 1, said exothermic material having a burning temperature of about 3,000 P.

3. The mold of claim 2, said exothermic material being composed essentially of iron oxide, aluminum, and sand.

4. The mold of claim 1, said side wall being progressively thicker from said bottom wall to said top wall.

5. The mold of claim 1, said top wall resting directly upon said side wall and extending laterally substantially beyond said mold cavity.

6. A process for casting a steel ingot having long columnar grains from bottom to top, comprising the steps of pouring molten steel into the open top of a mold cavity having its sides completely closed by exothermic material with a burning temperature of about 3,000 F. and its bottom closed by heat-conductive material fixed with re spect to said sides, completely closing said cavity by placing an imperforate cover of said exothermic material on top of said cavity directly upon and continuous with the closed exothermic sides of said cavity to seal said cavity, whereby said exothermic material burns and prevents loss of heat from said steel through the sides and top of said cavity, and withdrawing heat from said 'steel thrdugh 'sa'id heat-conductive material until said steel has solidified.

References Cited by the Examiner UNITED STATES PATENTS Roth 22+212 XR Roth 22-.-74 XR Snook 224-200 Bedilion 22216 10 Eldred 22200.1 Brennan 22216 Urmetz 22 -147 9 2,875,483 3/59 Hughes 2274 XR 2,951,272 9/60 Kiesler 222 12 XR FOREIGN PATENTS 3 67,615 2/ 32 Great Britain.

66 1,727 11/51 Great Britain.

698,303 10/ 5 3 Great Britain.

726,670 3 /5 5 Great Britain.

MARCUS U. LYONS, Primary Examiner.

WINSTON A. DOUGLAS, RAY K. WINDHAM, MICHAEL V. BRINDISI, NEDWIN BERGER, ROBERT F WHITE, Examiners. 

1. A MOLD FOR USE IN CASTING A STEEL INGOT, COMPRISING FIXED BOTTOM AND SIDE WALLS AND A SEPARABLE TOP WALL COMPLETELY ENCLOSING A MOLD CAVITY THEREBETWEEN, SAID BOTTOM WALL BEING FORMED OF HEAT CONDUCTIVE MATERIAL, SAID SIDE AND TOP WALLS BEING FORMED OF IMPERFORATE EXOTHERMIC MATERIAL WHICH EXTENDS CONTINUOUSLY OVER THE SIDES AND TOP OF SAID CAVITY WHEN SAID TOP WALL IS IN PLACE, WHEREBY MOLTEN STEEL MAY BE POURED INTOSAID MOLD WITH THE TOP WALL REMOVED AND THEN SAID TOP WALL MAY BE SET IN PLACE TO SEAL SAID MOLD CAVITY, THE COMBUSTION OF SAID EXOTHERMIC MATERIAL PREVENTING LOSS OF HEAT THROUGH SAID TOP AND SIDE WALLS TO ENSURE UNIDIRECTIONAL SOLIDIFICATION OF SAID INGOT AS HEAT IS WITHDRAWN THROUGH SAID BOTTOM WALL. 